The recycling and processing of waste paper are nowadays the most important processes for the recovery of raw materials for the production of new paper. Compared to paper products made from virgin fiber, the recycling of waste paper has a substantially lower environmental impact. Nevertheless, the paper industry is one of the most energy-intensive industries. In this work, stock preparation processes are optimized taking account of sustainability criteria. In addition to the fine screening process and flotation process, the dispersion process is examined in the stock preparation plants of two industry partners. At the same time, these three sub-processes are the most important processes that a pulp must go through in stock preparation. In this work, models of these basic processes are developed which carry parameters for the calculation of the relevant sustainability indicators. These parameters include, for example, the energy and water consumption, as well as the emergence of non-recyclable waste materials.